The present invention relates generally to circuits that include differential amplifiers as well as op amps. Specifically, the invention describes a method for reducing noise and thereby increasing dynamic range of an amplifier by adjusting the common mode of the feedback/input signal.
Since the existence of digital signal processing, A/D converters have been playing a very important role as interfaces between the analog and digital worlds. They perform the digitalization of analog signals at a fixed time period (frequency), i.e., the speed of A/D converters. The fixed time period is generally specified by the application based upon, for example, the Nyquist Sampling Theorem. As well known in the art, the Nyquist Sampling Theorem states that the highest frequency that can be accurately represented without aliasing is one-half of the sampling rate.
Since algorithms can be implemented very inexpensively in the digital domain and if the samples acquired satisfy the Nyquist Sampling Theorem, signals can be reconstructed perfectly after the digital signal processing. Hence, the A/D converter acts as a bridge between two domains and its accuracy is very critical to the performance of the system. In order to maintain optimal accuracy, noise sources must be kept to a minimum.
In this regard an RC integrator (commonly used in sigma-delta A/D converters) that includes a noise shaper that represents a particularly desirable approach for analog circuit design. The noise shaper typically includes CMOS transistors, which forces the input voltage Vin to have a voltage swing less than 5 volts. However, it is not uncommon (especially in power applications) for the output voltage Vout to be on the order of 50 volts. In power noise shaping processors, the early stages (i.e., which typically include a frequency selective network and/or a comparator) generally operate off of a supply that is less than the 50V power stage. Often 5V or +/xe2x88x925V is used for these stages. For these applications, an attenuation circuit must be used to reduce the signal swing at the input to less than +/xe2x88x925V.
For example, FIG. 1 shows an unbalanced output signal 100 from a noise shaped differential amplifier. Unfortunately, however, since a common mode signal is simply a division of the differential signal, a resulting common mode signal component 102 reduces the dynamic range of the amplifier by limiting the lowest allowed gain.
Therefore it would be advantageous to reduce the common mode signal of a noise shaping circuit resulting thereby in a reduced noise component and a concomitant increase in dynamic range.
The present invention relates to a method and apparatus for reducing the common mode signal of the noise shaping amplifier. In one embodiment, this reduced common mode signal results in a reduced noise component in the overall noise of the noise shaping amplifier and a concomitant increase in dynamic range.
In one aspect, a noise shaped differential amplifier having a reduced common mode signal in accordance with an embodiment of the invention is described. In the described embodiment, the noise shaped differential amplifier includes a noise shaper having a common mode signal controlled by an attenuation operational amplifier that is coupled to a voltage divider circuit and a sense resistor divider. In this arrangement, the attenuation operational amplifier controls a virtual ground applied to the sense resistor divider.
By controlling the virtual ground applied to the sense resistor divider, the attenuation operational amplifier controls the common mode signal of the noise shaping amplifier and allows a lower gain without clipping of internal nodes. In this way, the noise generated by the noise shaping amplifier is substantially reduced. It should noted that the attenuation operational amplifier only controls the voltage divider circuit (which is only operating on the common mode signal) and does not act as an additional source of noise in the noise shaping amplifier.